1. Field of the Invention
This patent application relates to spatially distributed sensor systems, and more particularly to spatially distributed, point-locating, intrusion-sensing optical fiber systems.
2. Discussion of Related Art
Fiber optic cable is well suited for distributed sensing of effects such as temperature and pressure. It's also ideally suited for sensing movement/vibration of the fiber, making it applicable for sensing intruders. Typical applications are found in security for perimeters, pipelines, rail, bridges, and other structures.
The simplest distributed sensors provide information about disturbances along the length of the fiber, but don't discriminate regarding their locations. FIG. 1 illustrates a typical sensor, based on modal interference in a multimode optical fiber. In this sensor, light travels along many modes in the optical fiber. Because the light is coherent, there is optical interference between the modes, resulting in a speckle pattern which is familiar to all who work with coherent radiation. Disturbances in the fiber result in strain that causes time-varying differential optical path lengths among the different modes. Because of these differential path lengths, disturbances of the fiber result in time variation in the speckle pattern. Thus, the sensor works by monitoring the speckle pattern, and watching for instances when the speckle pattern flickers.
The modalmetric sensor described in FIG. 1 is very sensitive and well suited for detecting disturbances along the fiber, but it doesn't give any information about the locations or the numbers of disturbances.
Other distributed fiber sensors provide information about the location and number of disturbances, but they are considerably more complicated and expensive than the modalmetric method illustrated in FIG. 1 (and more expensive and complicated than most other distributed sensors that don't provide location information). Some point-locating sensors use wavelength scanning and bimodal fiber (the dual modes may be spatial or polarization). Still other techniques use optical time-domain reflectometry and highly coherent light sources. The following list is a representative summary of various techniques reported in the literature, describing how to measure the time and location of an intrusion using a distributed fiber-optic sensor.
Stress-Location Measurement Along an Optical Fiber by Synthesis of Triangle-Shaped Optical Coherence Function, Kazuo Hotate, Xueliang Song, and Zuyuan He, IEEE Photonics Technology Letters, VOL. 13, NO. 3, MARCH 2001.
Large scale sensing arrays based on fiber Bragg gratings, M. G. Shlyagin1, I. Márquez Borbón1, V. V. Spirin1, R. Lopez1,E. A. Kuzin2, and M. May Alarcon2, Proceedings of SPIE Vol. 4578 (2002).
Distributed Fiber-Optic Stress-Location Measurement by Arbitrary Shaping of Optical Coherence Function, Zuyuan He, and Kazuo Hotate, Journal of Lightwave Technology, VOL. 20, NO. 9, SEPTEMBER 2002
Distributed Measurement of Strain using Optical Fibre Backscatter Polarimetry, A. J. Rogers, Department of electronic engineering, King's College, London.
Effect of the finite extinction ratio of an electro-optic modulator on the performance of distributed probe-pump Brillouin sensor systems, Shahraam Afshar V., Graham A. Ferrier, Xiaoyi Bao, and Liang Chen, Optics Letters/Vol. 28, No. 16/Aug. 15, 2003
Studies on a Few-Mode Fiber-Optic Strain Sensor Based on LP01-LP02 Mode Interference, Aran Kumar, Nitin K. Goel, and R. K. Varshney, Journal of Lightwave Technology, VOL. 19, NO. 3, MARCH 2001
Fiber-optic sensor using a tandem combination of a multimode fiber and a self-pumpedphase conjugator, Norman S. K. Kwong, Optics Letters/Vol. 14, No. 11/Jun. 1, 1989
Research of the distributed fiber optic pressure sensor, LU Haibao' CHU Xingchun LUO Wusheng SHEN Tingzheng YANG, Huayong National University of Defense Technology Dept. of Mechatronics Engineering and Instrument, Changsha Hunan 41 0073 China
Hybrid fiber-optic sensor using true heterodyne measurement techniques, David L. Mazzoni, Kyuman Cho, and Christopher C. Davis, Optics Letters/Vol. 16, No. 8/Apr. 15, 1991
A Novel Fiber Optic Sensor Array Based on the Sagnac Interferometer, Benjamin J. Vakoc, Michel J. F. Digonnet, and Gordon S. Kino, Part of the SPIE Conference on Fiber Optic Sensor Technology and Applications, Boston, Mass., September 1999, 276 SPIE Vol. 3860
Polarimetric and intermodal interference sensitivity to hydrostatic pressure, temperature, and strain of highly birefringent optical fibers, Wojtek J. Bock and Tinko A. Eftimov, Nov. 15, 1993/Vol. 18, No. 22/Optics Letters
Distributed optical-fiber sensor for spatial location of mode coupling by using the optical Kerr effect, I. Cokgor, V. A. Handerek, and A. J. Rogers, May 1, 1993/Vol. 18, No. 9/Optics Letters
Fiber-optic distributed sensing by a two-loop Sagnac interferometer, Xiaojun Fang, Optics Letters/Vol.21, No. 6/Mar. 15, 1996,
The last two references are illustrative of the typical design for point-locating, distributed fiber-optic sensors. FIG. 2 illustrates the construction of a point-locating sensor that uses the optical Kerr effect (this figure was taken from the paper by Cokgor, Handerek, and Rogers). Although this technique works well in the laboratory, and has very good spatial resolution, it is composed of expensive components that make it difficult to commercialize in an inexpensive and practical product.
In addition to simplicity, reliability, and low cost, an important practical requirement for any distributed optical sensor used in security applications is the ability to determine the locations of multiple intruders at one time. This requirement adds further constraints on the types of sensors that can be used. This is explained succinctly in a paper by Ilkka Alasaarela, Pentti Karioja, and Harri Kopola, titled Comparison of distributed fiber optic sensing methods for location and quantity information measurements, and published in Opt. Eng. 41(1) 181-189 (January 2002).
Furthermore, if measurement is limited to one perturbation at a time, the possibilities and device requirements become even more flexible. In this case, interferometric techniques are valid altematives—for vehicle location and fire detection, for example. Interferometric sensors can be used for measuring time-varying disturbances or impacts, which modulate the phase of the light inside the fiber. Actually, interferometric measurements are possible only when there is just one perturbation affecting the fiber loop. Therefore, their use is limited. Their advantages, on the other hand, include short measurement time, the applicability of short-coherence-length sources, and the possibility of simple construction.
The limitation of being able to find only one intrusion at a time is a serious one for technologies that use interferometric techniques. A marketable solution for security applications should be able to isolate multiple intrusions at one time, since the inability to isolate multiple intrusions at one time means the sensor's point-location capability can be defeated by simply vibrating the fiber near one end.
The problem of multiple events is also coupled to sensitivity. It's considerably more difficult to detect multiple simultaneous intrusions with good sensitivity than to achieve the same sensitivity while detecting a single intrusion. Fiber-optic sensors that provide high sensitivity and good location accuracy are also more difficult to build, and are typically expensive to manufacture. This is especially true when there is a requirement to locate multiple sources of disturbance with high spatial accuracy. Typically the simultaneous requirements for high spatial accuracy and high resolution are mutually inconsistent goals (see FIGS. 3 and 4).
What is needed is an inexpensive, sensitive, distributed fiber-optic sensor that can simultaneously identify the locations of multiple disturbances.